Gravitational waves offer glimpse into the past – but will we ever catch ripples from the Big Bang?

Einstein was right – changes in gravity do spread as waves through space. The LIGO experiment detected such waves from a collision between two black holes with masses of about 36 and 29 times that of the sun (described as 36 and 29 "solar masses"). But the merger of these 65 solar masses in total created a remnant of just 62 – so what happened to the other three? These were used to power the burst of gravitational waves, in a spectacular demonstration of Einstein's famous formula, E=Mc2, where mass and energy are equivalent.

This is only the beginning. Now that we know how to measure gravitational waves, we can use experiments like LIGO to learn about events in the cosmos that we have never been able to see before, such as mergers of supermassive black holes in the early universe. But how far back can we go? What about "primordial" gravitational waves from the birth of the universe itself – will LIGO's discovery help us catch those?

Looking back in time

Although the masses involved in this event are large by stellar standards, they are dwarfed by the supermassive black holes that astronomers believe are present at the centre of almost every galaxy. Our own galaxy, the Milky Way, hosts a hole of about 4m sun masses, detected through the motions of stars orbiting it. Even this is fairly insignificant compared with the holes of up to tens of billions of sun masses thought to be at the centre of the largest galaxies.

There are many things astronomers want to know about these supermassive black holes. We currently see them through the vast amounts of electromagnetic radiation, like visible light and X-rays, produced as gas falls into them. We know that this process helps them grow but it is nevertheless mysterious – most of the gas in galaxies moves too fast or is too far away for the black holes to capture it. So how could they get so big?

Illustration of galaxy with jets from a supermassive black hole. ESA/Hubble, CC BY-SA

It could be that collisions between these supermassive holes helped to grow them, especially when they were relatively young and had not yet gained much gas. However, a collision between two supermassive black holes can probably only happen if the two galaxies hosting them collide and merge too. This is an inherently rare event in the nearby universe, as galaxies are far away from each other. But it must have been much more common soon after the universe was born in the Big Bang, when galaxies were much closer together.

So detecting gravitational waves from such collisions means looking back in time – observing the most distant galaxies. Light from these galaxies set off on its journey to us only a relatively short time after the Big Bang. This could give us direct clues about how important these events were in growing supermassive black holes early in their lives. This is relevant to our own existence – the electromagnetic radiation thrown out as black holes grow has had a major effect on shaping the galaxies in which stars and planets, including our own, live peaceful lives.

To make such observations will require detectors with sizes far larger than the 4km arms of LIGO. The proposed eLISA experiment will put three satellites into orbit as an equilateral triangle with sides longer than the distance from the Earth to the moon.

The problem with primordial waves

But even supermassive black hole collisions are not the ultimate goal. The Big Bang, and particularly the epoch of very rapid expansion dubbed inflation – which many experts believe took place very soon after – must have involved enormous masses moving with almost light speed. This means that they must have produced powerful gravitational waves. However, the most powerful signal comes from masses whose size is comparable to the scale of the universe itself. Since gravitational radiation has a typical wavelength larger than the masses emitting it, the "wavelength" of this radiation is itself similar to the entire size of the universe. So LIGO, or any other experiment that is smaller than the universe, will not be able to detect it.

Detecting these waves must probably be done indirectly by observing their effects on cosmic microwave background radiation (CMB) – the radiation left over from the Big Bang.

When light waves vibrate in a certain direction, we say that the light has a specific polarisation. If gravitational waves were present at the time when the CMB was born, they should leave behind a unique swirly pattern –- a curling in the polarisation of the light –- dubbed "B modes". A result based on B modes was claimed a few years ago, but it turned out the signal had just been caused by cosmic dust. This is just one of the many competing effects that can distort the CMB polarisation, showing just how hard it will be to detect the true signal.

The stakes are incredibly high. A positive result could give evidence for the popular inflation theory, and offer explanations for several puzzling features of the universe, such as why the distribution of matter is so homogeneous. Although finding such a signal is an enormous challenge, so was the direct detection of gravitational waves when first proposed half a century ago.

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20 comments

hm, aren't we losing mass/energy into spacetime? Something that was apparently a massive object is converted into energy driving the waves. And I don't see how that energy can be returned into the universe because spacetime is not an object inside it.

Of course someone at your publication should know that this "discovery of gravitational waves" is just bs, right? What these guys have developed is a new way to "hear" something....exactly what they are guessing. And it probably is no more than feed-back static. Guessing. Because if you apply logic to this you'd see what they are guessing at is impossible.And you publications are buying right into it because you have idiots for reporters, English majors for editors and media-moguls for owners.

They might have passed our earth ALREADY; Probably, They are the ones that caused disappearance of DINOSAURS.Joke: Stripped their flesh and took away to some other planets....leaving their bones behind here!

Could space-time itself be the gravitational effect of the Big Bang?Gravitational waves travel at the speed of light.Are gravitational waves affected by curved space-time the way light is affected?When Jupiter is 33 light-minutes away Do we 'feel' the space time curvature where Jupiter was 33 minutes ago?The Cal Tech video shows the two event horizon jump many kilometers in less than .001 second. Do black holes make the final merge as a quantum leap?

The use of the words [must have] are the clear give away here that the Big Bang is not the reality that people are making it out to be. Since it wasn't observed and since it cannot be repeated or confirmed it remains sheer speculation of the highest kind. So far the actual, real, observed evidence does NOT support the idea of a Big Bang but in fact quite clearly contradicts it.For instance we find fully matured galaxies at the very extremities of observations - plus those galaxies contain mixture of socalled old as well as young stars. We also see the enigma of blue stars everywhere, defiantly not showing the required evidence of so called recent and continuing star birth.That is just to name a few enigmas.

How do 65 solar masses merge into 62 solar masses and how do three solar masses convert into gravitational waves in the final second?Is it due to the motion of the mass through space-time or due to the intense gravity field? If due to motion, is all mass in motion in the Universe loosing energy to gravitational waves? If dark matter or dark energy is correlated to mass than what part did the dark matter or energy play in the black hole binary merge. So much to learn. The work of scientists is much appreciated.

So, if I understand it correctly (doubtful, LOL), a gravity wave is a traveling ripple/curvature in space time. So, can you deliver energy directly into spacetime, as a medium, much like you can Air, water, etc?

Or does this merely confirm that the force of gravity travels at the speed of light. For example, if the sun vanished, would the planets continue to orbit, powered by the gravity waves they are still getting (travelling at the speed of light from sun to the planet), until there were no more gravity waves?

I dont know, things seem more related to the aether model to me, there is a base substance to space to accept/propogate gravity waves.

... Are gravitational waves affected by curved space-time the way light is affected?

No, but gravity waves must display a similar superposition effect. If you are at the spatial and temporal intersection of two trains of gravitational waves, the gravitational wave effect at that location will be the algebraic, linear sum of the two waves.

When Jupiter is 33 light-minutes away Do we 'feel' the space time curvature where Jupiter was 33 minutes ago?

Yes, but the effects are practically negligible.

The Cal Tech video shows the two event horizon jump many kilometers in less than .001 second. Do black holes make the final merge as a quantum leap?

I think not, but I am not qualified to say. I base my opinion on the the existence of the ringdown waveform, which can be modelled as a distorted black hole, becoming more and more symmetrical and emitting less and less energy, gravitationally. That doesn't strike me as a quantum merger.

They might have passed our earth ALREADY; Probably, They are the ones that caused disappearance of DINOSAURS.Joke: Stripped their flesh and took away to some other planets....leaving their bones behind here!

Bing Bang came first; Dinosaurs Next. But, yet humongous gravitational waves MUST have come to knock off Mighty Dinosaurs too...later on!Easy to Check it; Take a Biting Dog..So Many have those Pests and they will be ready to part with them. Create enough amount of Gravitational Waves & Subject it to those. That will PROVE whether Dinosaurs' Extinction on this planet was because of Gravitational Waves.LIGOs are everywhere nowadays and are also going to spring up Everywhere too!

Of course someone at your publication should know that this "discovery of gravitational waves" is just bs, right? What these guys have developed is a new way to "hear" something....exactly what they are guessing. And it probably is no more than feed-back static. Guessing. Because if you apply logic to this you'd see what they are guessing at is impossible.

And you publications are buying right into it because you have idiots for reporters, English majors for editors and media-moguls for owners.

I highly recommend the following article for you. It's written with the lay person in mind:

@vp, Benny, krundoloss: "And I don't see how that energy can be returned into the universe", "energy into gravity?", "energy into spacetime, as a medium...?",

The generated gravitational waves transfer energy out (and that energy can do work, in the LIGO detector say). You don't need to speculate in what spacetime is, because waves is a result of special relativity (since there must be a universal speed limit and no immediate action at a distance).

Especially there is no "aether", it is a century old observation: https://en.wikipe...element) (On the contrary, relativity also predicts why there isn't one.)

@JB85: Too many questions for a limited comment space. Let us take one: "Could space-time itself be the gravitational effect of the Big Bang?"

That depends on what you mean by "big bang", there are numerous definitions. But we don't need that here since the universe is zero energy so have no self gravity, it is flat on average. What spacetime and gravity is is an open question. General relativity connects them, but doesn't predict all the physics.

@betterexists: "Serious question: [superstition] or [science]?"

Doesn't look like a serious question to me. Ask about the science, and you will get serious answers.

@OCC: "Why does the universe require a beginning?"

Who says it does? We can't observe the whole era of inflation.

If an article claims an "initial big bang" before inflation, instead of the hot big bang after inflation that we can see, it is speculative (and mildly confusing).

Since gravitational radiation has a typical wavelength larger than the masses emitting it, the "wavelength" of this radiation is itself similar to the entire size of the universe.

Gravitational radiation? Where do these insane ideas come from? There is no such a thing as gravitational radiation. Gravity doesn't "propagate" as waves. Gravity is not in the optical spectrum. Gravity is not an electro-magnetic wave. Gravity does not have velocity. Masses do not "emit" gravity.

This article is so full of crap. Some out-of-work Encyclopedia Britannica writer must have made all this up.

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